It can be the on-off top. But it can also be too many shims in the powertube or something else causing the bolt stem vent hole to be at the edge of the white carrier oring. When the pressure is high in a fully charged chamber it pushes the oring hard against the surface, but when the pressure is low in the case of a mostly empty chamber, the oring surface isn't pressed hard against the powertube and it doesn't hold air as well when it is at the edge of the vent hole. The air can escape through the smaller sealing surface at that point.
Check both options. Its an easy fix in both situations.
Except for the Automag in front, its usually the man behind the equipment that counts.